MINOR: quic: remove qc from quic_rx_packet

MINOR: quic: remove qc from quic_rx_packet

quic_rx_packet struct had a reference to the quic_conn instance. This is
useless as qc instance is always passed through function argument. In
fact, pkt.qc is used only in qc_pkt_decrypt() on key update, even though
qc is also passed as argument.

Simplify this by removing qc field from quic_rx_packet structure
definition. Also clean up qc_pkt_decrypt() documentation and interface
to align it with other quic-conn related functions.

This should be backported up to 2.7.